Nanoporous ceramic membranes capable of selectively filtering molecules offer numerous advantages over their polymeric counterparts, including enhanced chemical stability and lower fouling. High-temperature sintering and calcination of ceramic membranes are used to improve durability and reduce grain boundaries; otherwise ceramic membranes break under the compression required to create air- and liquid-tight seals. Unfortunately, these high-temperature processing steps dramatically increase the cost of the membrane to the point they often cannot be implemented commercially.
The ability to create ceramic selective membranes can be of especially significant benefit to redox flow batteries (RFB). RFBs provide a promising solution to grid-scale energy storage needs. Unlike conventional solid-state batteries, the electrolytes in RFBs are stored in external tanks and are pumped through the cell stack of the battery. Thus, the RFB possesses several attractive qualities, such as ease of scalability, long service life, and the separation of power and energy. Unfortunately, the current high cost of RFBs has hindered their ability to be widely commercialized. The membrane contributes significantly to this high cost, where it can account for up to 40% of the total capital cost. In these applications, membranes must be able to transport charge balancing ions and prevent the crossover of active species. The poor ion selectivity of commercial membranes has led to an emphasis on the all-vanadium RFB (VRFB), which can mitigate the detrimental effect of crossover due to its symmetric electrolyte composition. Instead of a permanent loss of capacity, crossover only leads to a loss of efficiency in VRFBs. However, vanadium is expensive and can result in as much as 50% of the total capital costs of an RFB. The U.S. Department of Energy has listed a target cost of $100/kWh for RFBs, yet VRFBs have an estimated capital cost of $447/kWh.
As the first true RFB, the Iron-Chromium redox flow battery (ICRFB) is very attractive in terms of its cost effectiveness, comprising cheap and abundant active materials. It has a standard overall cell potential of 1.18 V with the reactions occurring within the battery are listed below.Positive Electrode: Fe2+⇄Fe3++e−E0=+0.77V  (1)Negative Electrode: Cr3++e−⇄Cr2+E0=−0.41V  (2)Overall: Fe2++Cr3+⇄Fe3++Cr2+E0=+1.18V  (3)
Despite the cost effectiveness of the ICRFB, it is prone to significant membrane crossover of the active species that results in irreversible capacity decay. This same capacity occurs for many other inexpensive RFB chemistries and has largely prevented their commercialization.
There is still a need for a selective sol-gel membrane that can reduce flow battery costs and enable new flow battery chemistries.